Method for receiving a mac ce for contention-based pusch in a wireless communication system and a device therefor

ABSTRACT

The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for d receiving a MAC CE for contention-based PUSCH in a wireless communication system, the method comprising: configuring a CB grant and a CB-RNTI associated with the CB grant; transmitting a first MAC PDU including an identifier of the UE on the CB grant; receiving a CB MAC CE indicated by a PDCCH addressed to the CB-RNTI in response to the MAC PDU transmission; determining that the first MAC PDU transmission is failed if the identifier of the UE isn&#39;t included in the MAC CE; and transmitting a second MAC PDU including the identifier of the UE on the CB grant.

TECHNICAL FIELD

The present invention relates to a wireless communication system and, more particularly, to a method for receiving a MAC CE for contention-based PUSCH in a wireless communication system and a device therefor.

BACKGROUND ART

As an example of a mobile communication system to which the present invention is applicable, a 3rd Generation Partnership Project Long Term Evolution (hereinafter, referred to as LTE) communication system is described in brief.

FIG. 1 is a view schematically illustrating a network structure of an E-UMTS as an exemplary radio communication system. An Evolved Universal Mobile Telecommunications System (E-UMTS) is an advanced version of a conventional Universal Mobile Telecommunications System (UMTS) and basic standardization thereof is currently underway in the 3GPP. E-UMTS may be generally referred to as a Long Term Evolution (LTE) system. For details of the technical specifications of the UMTS and E-UMTS, reference can be made to Release 7 and Release 8 of “3rd Generation Partnership Project; Technical Specification Group Radio Access Network”.

Referring to FIG. 1, the E-UMTS includes a User Equipment (UE), eNode Bs (eNBs), and an Access Gateway (AG) which is located at an end of the network (E-UTRAN) and connected to an external network. The eNBs may simultaneously transmit multiple data streams for a broadcast service, a multicast service, and/or a unicast service.

One or more cells may exist per eNB. The cell is set to operate in one of bandwidths such as 1.25, 2.5, 5, 10, 15, and 20 MHz and provides a downlink (DL) or uplink (UL) transmission service to a plurality of UEs in the bandwidth. Different cells may be set to provide different bandwidths. The eNB controls data transmission or reception to and from a plurality of UEs. The eNB transmits DL scheduling information of DL data to a corresponding UE so as to inform the UE of a time/frequency domain in which the DL data is supposed to be transmitted, coding, a data size, and hybrid automatic repeat and request (HARQ)-related information. In addition, the eNB transmits UL scheduling information of UL data to a corresponding UE so as to inform the UE of a time/frequency domain which may be used by the UE, coding, a data size, and HARQ-related information. An interface for transmitting user traffic or control traffic may be used between eNBs. A core network (CN) may include the AG and a network node or the like for user registration of UEs. The AG manages the mobility of a UE on a tracking area (TA) basis. One TA includes a plurality of cells.

Although wireless communication technology has been developed to LTE based on wideband code division multiple access (WCDMA), the demands and expectations of users and service providers are on the rise. In addition, considering other radio access technologies under development, new technological evolution is required to secure high competitiveness in the future. Decrease in cost per bit, increase in service availability, flexible use of frequency bands, a simplified structure, an open interface, appropriate power consumption of UEs, and the like are required.

DISCLOSURE Technical Problem

An object of the present invention devised to solve the problem lies in a method and device for receiving a MAC CE for contention-based PUSCH in a wireless communication system. The technical problems solved by the present invention are not limited to the above technical problems and those skilled in the art may understand other technical problems from the following description.

Technical Solution

The object of the present invention can be achieved by providing a method for User Equipment (UE) operating in a wireless communication system as set forth in the appended claims.

In another aspect of the present invention, provided herein is a communication apparatus as set forth in the appended claims.

It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

Advantageous Effects

The invention is that the UE receives a Contention-based MAC CE by including the UE identifier of the successfully identified UE in order to inform the UEs using the same CB grant of the successfully identified UE on the CB grant when the UE transmits a MAC PDU by using a contention based grant.

It will be appreciated by persons skilled in the art that the effects achieved by the present invention are not limited to what has been particularly described hereinabove and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principle of the invention.

FIG. 1 is a diagram showing a network structure of an Evolved Universal Mobile Telecommunications System (E-UMTS) as an example of a wireless communication system;

FIG. 2A is a block diagram illustrating network structure of an evolved universal mobile telecommunication system (E-UMTS), and FIG. 2B is a block diagram depicting architecture of a typical E-UTRAN and a typical EPC;

FIG. 3 is a diagram showing a control plane and a user plane of a radio interface protocol between a UE and an E-UTRAN based on a 3rd generation partnership project (3GPP) radio access network standard;

FIG. 4 is a view showing an example of a physical channel structure used in an E-UMTS system;

FIG. 5 is a block diagram of a communication apparatus according to an embodiment of the present invention;

FIG. 6 is an example for a diagram for performing contention based transmission;

FIG. 7 is an example for a diagram for performing contention based SR procedure;

FIG. 8 is an example for a diagram for performing contention based transmission;

FIG. 9 is a diagram for MAC structure overview in a UE side;

FIG. 10 is a diagram for Scheduling-request transmission;

FIG. 11 is a diagram for signaling of buffer status and power-headroom reports; and

FIG. 12 is a conceptual diagram for receiving a MAC CE for contention-based PUSCH in a wireless communication system according to embodiments of the present invention.

BEST MODE

Universal mobile telecommunications system (UMTS) is a 3rd Generation (3G) asynchronous mobile communication system operating in wideband code division multiple access (WCDMA) based on European systems, global system for mobile communications (GSM) and general packet radio services (GPRS). The long-term evolution (LTE) of UMTS is under discussion by the 3rd generation partnership project (3GPP) that standardized UMTS.

The 3GPP LTE is a technology for enabling high-speed packet communications. Many schemes have been proposed for the LTE objective including those that aim to reduce user and provider costs, improve service quality, and expand and improve coverage and system capacity. The 3G LTE requires reduced cost per bit, increased service availability, flexible use of a frequency band, a simple structure, an open interface, and adequate power consumption of a terminal as an upper-level requirement.

Hereinafter, structures, operations, and other features of the present invention will be readily understood from the embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Embodiments described later are examples in which technical features of the present invention are applied to a 3GPP system.

Although the embodiments of the present invention are described using a long term evolution (LTE) system and a LTE-advanced (LTE-A) system in the present specification, they are purely exemplary. Therefore, the embodiments of the present invention are applicable to any other communication system corresponding to the above definition. In addition, although the embodiments of the present invention are described based on a frequency division duplex (FDD) scheme in the present specification, the embodiments of the present invention may be easily modified and applied to a half-duplex FDD (H-FDD) scheme or a time division duplex (TDD) scheme.

FIG. 2A is a block diagram illustrating network structure of an evolved universal mobile telecommunication system (E-UMTS). The E-UMTS may be also referred to as an LTE system. The communication network is widely deployed to provide a variety of communication services such as voice (VoIP) through IMS and packet data.

As illustrated in FIG. 2A, the E-UMTS network includes an evolved UMTS terrestrial radio access network (E-UTRAN), an Evolved Packet Core (EPC) and one or more user equipment. The E-UTRAN may include one or more evolved NodeB (eNodeB) 20, and a plurality of user equipment (UE) 10 may be located in one cell. One or more E-UTRAN mobility management entity (MME)/system architecture evolution (SAE) gateways 30 may be positioned at the end of the network and connected to an external network.

As used herein, “downlink” refers to communication from eNodeB 20 to UE 10, and “uplink” refers to communication from the UE to an eNodeB. UE 10 refers to communication equipment carried by a user and may be also referred to as a mobile station (MS), a user terminal (UT), a subscriber station (SS) or a wireless device.

FIG. 2B is a block diagram depicting architecture of a typical E-UTRAN and a typical EPC.

As illustrated in FIG. 2B, an eNodeB 20 provides end points of a user plane and a control plane to the UE 10. MME/SAE gateway 30 provides an end point of a session and mobility management function for UE 10. The eNodeB and MME/SAE gateway may be connected via an S1 interface.

The eNodeB 20 is generally a fixed station that communicates with a UE 10, and may also be referred to as a base station (BS) or an access point. One eNodeB 20 may be deployed per cell. An interface for transmitting user traffic or control traffic may be used between eNodeBs 20.

The MME provides various functions including NAS signaling to eNodeBs 20, NAS signaling security, AS Security control, Inter CN node signaling for mobility between 3GPP access networks, Idle mode UE Reachability (including control and execution of paging retransmission), Tracking Area list management (for UE in idle and active mode), PDN GW and Serving GW selection, MME selection for handovers with MME change, SGSN selection for handovers to 2G or 3G 3GPP access networks, Roaming, Authentication, Bearer management functions including dedicated bearer establishment, Support for PWS (which includes ETWS and CMAS) message transmission. The SAE gateway host provides assorted functions including Per-user based packet filtering (by e.g. deep packet inspection), Lawful Interception, UE IP address allocation, Transport level packet marking in the downlink, UL and DL service level charging, gating and rate enforcement, DL rate enforcement based on APN-AMBR. For clarity MME/SAE gateway 30 will be referred to herein simply as a “gateway,” but it is understood that this entity includes both an MME and an SAE gateway.

A plurality of nodes may be connected between eNodeB 20 and gateway 30 via the S1 interface. The eNodeBs 20 may be connected to each other via an X2 interface and neighboring eNodeBs may have a meshed network structure that has the X2 interface.

As illustrated, eNodeB 20 may perform functions of selection for gateway 30, routing toward the gateway during a Radio Resource Control (RRC) activation, scheduling and transmitting of paging messages, scheduling and transmitting of Broadcast Channel (BCCH) information, dynamic allocation of resources to UEs 10 in both uplink and downlink, configuration and provisioning of eNodeB measurements, radio bearer control, radio admission control (RAC), and connection mobility control in LTE_ACTIVE state. In the EPC, and as noted above, gateway 30 may perform functions of paging origination, LTE-IDLE state management, ciphering of the user plane, System Architecture Evolution (SAE) bearer control, and ciphering and integrity protection of Non-Access Stratum (NAS) signaling.

The EPC includes a mobility management entity (MME), a serving-gateway (S-GW), and a packet data network-gateway (PDN-GW). The MME has information about connections and capabilities of UEs, mainly for use in managing the mobility of the UEs. The S-GW is a gateway having the E-UTRAN as an end point, and the PDN-GW is a gateway having a packet data network (PDN) as an end point.

FIG. 3 is a diagram showing a control plane and a user plane of a radio interface protocol between a UE and an E-UTRAN based on a 3GPP radio access network standard. The control plane refers to a path used for transmitting control messages used for managing a call between the UE and the E-UTRAN. The user plane refers to a path used for transmitting data generated in an application layer, e.g., voice data or Internet packet data.

A physical (PHY) layer of a first layer provides an information transfer service to a higher layer using a physical channel. The PHY layer is connected to a medium access control (MAC) layer located on the higher layer via a transport channel. Data is transported between the MAC layer and the PHY layer via the transport channel. Data is transported between a physical layer of a transmitting side and a physical layer of a receiving side via physical channels. The physical channels use time and frequency as radio resources. In detail, the physical channel is modulated using an orthogonal frequency division multiple access (OFDMA) scheme in downlink and is modulated using a single carrier frequency division multiple access (SC-FDMA) scheme in uplink.

The MAC layer of a second layer provides a service to a radio link control (RLC) layer of a higher layer via a logical channel. The RLC layer of the second layer supports reliable data transmission. A function of the RLC layer may be implemented by a functional block of the MAC layer. A packet data convergence protocol (PDCP) layer of the second layer performs a header compression function to reduce unnecessary control information for efficient transmission of an Internet protocol (IP) packet such as an IP version 4 (IPv4) packet or an IP version 6 (IPv6) packet in a radio interface having a relatively small bandwidth.

A radio resource control (RRC) layer located at the bottom of a third layer is defined only in the control plane. The RRC layer controls logical channels, transport channels, and physical channels in relation to configuration, re-configuration, and release of radio bearers (RBs). An RB refers to a service that the second layer provides for data transmission between the UE and the E-UTRAN. To this end, the RRC layer of the UE and the RRC layer of the E-UTRAN exchange RRC messages with each other.

One cell of the eNB is set to operate in one of bandwidths such as 1.25, 2.5, 5, 10, 15, and 20 MHz and provides a downlink or uplink transmission service to a plurality of UEs in the bandwidth. Different cells may be set to provide different bandwidths.

Downlink transport channels for transmission of data from the E-UTRAN to the UE include a broadcast channel (BCH) for transmission of system information, a paging channel (PCH) for transmission of paging messages, and a downlink shared channel (SCH) for transmission of user traffic or control messages. Traffic or control messages of a downlink multicast or broadcast service may be transmitted through the downlink SCH and may also be transmitted through a separate downlink multicast channel (MCH).

Uplink transport channels for transmission of data from the UE to the E-UTRAN include a random access channel (RACH) for transmission of initial control messages and an uplink SCH for transmission of user traffic or control messages. Logical channels that are defined above the transport channels and mapped to the transport channels include a broadcast control channel (BCCH), a paging control channel (PCCH), a common control channel (CCCH), a multicast control channel (MCCH), and a multicast traffic channel (MTCH).

FIG. 4 is a view showing an example of a physical channel structure used in an E-UMTS system. A physical channel includes several subframes on a time axis and several subcarriers on a frequency axis. Here, one subframe includes a plurality of symbols on the time axis. One subframe includes a plurality of resource blocks and one resource block includes a plurality of symbols and a plurality of subcarriers. In addition, each subframe may use certain subcarriers of certain symbols (e.g., a first symbol) of a subframe for a physical downlink control channel (PDCCH), that is, an L1/L2 control channel. In FIG. 4, an L1/L2 control information transmission area (PDCCH) and a data area (PDSCH) are shown. In one embodiment, a radio frame of 10 ms is used and one radio frame includes 10 subframes. In addition, one subframe includes two consecutive slots. The length of one slot may be 0.5 ms. In addition, one subframe includes a plurality of OFDM symbols and a portion (e.g., a first symbol) of the plurality of OFDM symbols may be used for transmitting the L1/L2 control information. A transmission time interval (TTI) which is a unit time for transmitting data is 1 ms.

A base station and a UE mostly transmit/receive data via a PDSCH, which is a physical channel, using a DL-SCH which is a transmission channel, except a certain control signal or certain service data. Information indicating to which UE (one or a plurality of UEs) PDSCH data is transmitted and how the UE receive and decode PDSCH data is transmitted in a state of being included in the PDCCH.

For example, in one embodiment, a certain PDCCH is CRC-masked with a radio network temporary identity (RNTI) “A” and information about data is transmitted using a radio resource “B” (e.g., a frequency location) and transmission format information “C” (e.g., a transmission block size, modulation, coding information or the like) via a certain subframe. Then, one or more UEs located in a cell monitor the PDCCH using its RNTI information. And, a specific UE with RNTI “A” reads the PDCCH and then receive the PDSCH indicated by B and C in the PDCCH information.

FIG. 5 is a block diagram of a communication apparatus according to an embodiment of the present invention.

The apparatus shown in FIG. 5 can be a user equipment (UE) and/or eNB adapted to perform the above mechanism, but it can be any apparatus for performing the same operation.

As shown in FIG. 5, the apparatus may comprises a DSP/microprocessor (110) and RF module (transmiceiver; 135). The DSP/microprocessor (110) is electrically connected with the transciver (135) and controls it. The apparatus may further include power management module (105), battery (155), display (115), keypad (120), SIM card (125), memory device (130), speaker (145) and input device (150), based on its implementation and designer's choice.

Specifically, FIG. 5 may represent a UE comprising a receiver (135) configured to receive a request message from a network, and a transmitter (135) configured to transmit the transmission or reception timing information to the network. These receiver and the transmitter can constitute the transceiver (135). The UE further comprises a processor (110) connected to the transceiver (135: receiver and transmitter).

Also, FIG. 5 may represent a network apparatus comprising a transmitter (135) configured to transmit a request message to a UE and a receiver (135) configured to receive the transmission or reception timing information from the UE. These transmitter and receiver may constitute the transceiver (135). The network further comprises a processor (110) connected to the transmitter and the receiver. This processor (110) may be configured to calculate latency based on the transmission or reception timing information.

FIG. 6 is an example for a diagram for performing contention based transmission.

One simple but efficient method for latency reduction for typical internet traffic in an unloaded or partially-loaded network is Pre-allocation. Pre-allocation is a form of pre-scheduling that provides the UE with an opportunity to send UL packets without requiring the UE to send a scheduling request. The scheduling request procedure when in-sync takes 10 ms, and this makes it impossible for LTE to support the original RAN latency requirement of 10 ms round trip (2×5 ms one way) delay defined in the original LTE requirement specification 25.913 unless UL resources are pre-scheduled for the terminal.

Pre-allocation provides resource block grants to UEs in case they have something to transmit when those resource blocks are not used for actual traffic from other UEs. One possibility for the network is to trigger such pre-allocation of UL resources using downlink packets that are likely to require some acknowledgement from the UE (e.g. a Ping or TCP ACK). More generalized schemes can also be considered.

Note that Pre-allocation is different from other forms of pre-scheduling such as Semi-persistent scheduling. Pre-allocation uses the PDCCH to grant UL resources when not used by actual traffic. On the other hand semi-persistent scheduling provides a regular allocation to the UE without repetitive scheduling on the PDCCH.

Meanwhile, the goal with Contention Based (CB) transmission is to allow uplink synchronized UEs to transmit uplink data without sending Scheduling Request in advance. That would reduce both the latency and the signaling overhead. For small data packets, there could be a tradeoff point where a small packet is more efficiently transmitted on a CB channel, compared to a scheduled one.

A general property of CB channels is that the error rate increases, since data packets may collide with each other. Collisions reduce the maximum throughput of the channel and the throughput becomes sensitive to the offered load. If the offered load is allowed to increase beyond the channel capacity, the collision probability increases rapidly, the system becomes unstable and the throughput decreases. It is therefore of prime importance that CB transmissions do not interfere with Contention Free (CF) uplink transmissions, and that the eNB has effective and fast means of allocating the resources for CB transmission.

One way to achieve the above is to allow CB transmission only in uplink Resource Blocks that have not been reserved for CF uplink transmission. Dynamic assignment of uplink Resource Blocks for CB transmission can be achieved by using the Downlink Physical Control CHannel (PDCCH). By using the PDCCH, CB grants can be assigned to unused resources on a per subframe basis, so that scheduling of uplink CF transmissions is not affected. In this way, a static assignment of CB resources can be avoided, and CB resources can be dynamically assigned, depending on the uplink load.

Contention Based Radio Network Temporary Identifiers (CB-RNTI) are introduced to identify the CB uplink grants on the PDCCH. The CB uplink grants could have the same format as for Rel-8 UEs, i.e. specify Resource Blocks, Modulation and Coding Scheme and Transport Format to be used for the uplink CB transmission. Rel-10 UEs may listen for CB uplink grants addressed to these CB-RNTIs in addition to grants addressed to their dedicated C-RNTI. The available CB-RNTIs in a cell could be either broadcasted or signaled to each UE during RRC connection setup. The scheme is backwards compatible, since pre Rel-10 UEs would not decode the grants addressed to CB-RNTIs.

As a common resource is used, a unique UE identifier is needed in the MAC PDU to identify the UE. The C-RNTI MAC Control Element can be added to each MAC PDU transmitted on a CB uplink resource.

A UE should only be allowed to transmit on CB uplink grants if it does not have a dedicated CF grant. The UE should only be allowed to use CB resources for a limited number of subframes, to improve collision resolution. In parallel to the CB transmission, the UE can also transmit a Scheduling Request to request contention free resources. Note however that in order to maintain the single carrier uplink property, these cannot be transmitted in the same subframe.

The contention based transmission scheme could be as shown in FIG. 6.

Regarding FIG. 6, the eNodeB informs UE of available CB-RNTIs either by broadcast or dedicated signaling (A). The UE receives the CB-RNTIs and starts monitoring PDCCH for available CB grant (B). The eNodeB schedules a CB grant on the PDCCH (C), and the UE detects a CB grant and performs L2&L1 processing of the data to be transmitted (D). The UE transmits the data on PUSCH using the CB grant (E).

In the proposed form, the CB transmission is supported only for synchronized UEs. In this form, changes to the current specifications are expected to be small and would mainly affect the MAC and RRC specifications. As presented in section 3, there is a perceivable gain in e.g. TCP performance.

Extending the concept to also cover unsynchronized UEs would require substantial changes to the physical layer specifications. For unsynchronized UEs, the transmissions would not fit within the subframe borders, and there would be a need for guard times to avoid overlapping transmissions. Also some form of preamble would be needed to synchronize the eNB receiver. The gain of extending CB transmission scheme to unsynchronized UEs is expected to be small. The gain for synchronized UEs comes from repetitions of the 6 ms difference. For unsynchronized UEs, this would come only once per transaction, as after this the UE would be synchronized. Therefore, we do not consider CB transmissions from unsynchronized UEs to be a worthwhile solution.

FIG. 7 is an example for a diagram for performing contention based SR procedure.

In Rel-8, the SR resource and sequence is allocated to a UE via RRC signaling. Of course shorter SR periodicities come at the cost of higher PUCCH resource consumption. The theoretical SR capacity is 18 UEs per PRB, if 180 UEs are supported, the number of PRB equals 180/18=10. If 10 MHz bandwidth is assumed with 1 ms SR period, 20% resource will be used for SR, which is a heavy control channel burden. Hence we propose to consider sharing the SR resource among more than one UE.

FIG. 7 illustrates how SR could be shared. The eNB configures the same SR resource for several UEs through RRC signaling (S701). The UE(s) sends SR to the eNB using the configured SR resource (S703). If there is no collision SR, the eNB allocates a PUSCH grant (S705). The UE transmits the uplink data on the PUSCH (S707).

Two options can be considered to enable sharing SR.

Option1 is that the UL Grant is addressed to a new SR-RNTI (Shared SR RNTI), configured per group of sharing UEs. Option 2 is that PUCCH Format 1a and/or Format 1b is used for SR. For example, when Format 1a is used, 2 UEs can be identified; with for Format 1b, 4UEs can be identified. After the eNB receives the SR using format 1a and/or 1b, it can allocate a regular UL grant to the identified UE.

We now discuss the handling when more than one UE uses the same PUCCH-SR resource in a TTI (collision).

For option1, the eNB may not be able to tell when a PUCCH-SR collision happens; it grants resources for UL transmission and more than one UE uses it. The PUSCH transmission will fail. The eNB could in this case provide grants to the C-RNTIs of each UE sharing that resource or do nothing. The UE may send SR again if no UL grant is received after sending SR, but needs to apply some (random or UE specific) delay, to avoid continued collision with another UE that would have sent an SR at the same time. The efficiency of such solution depends on the collision probability and degree of robustness of the selected MCS in UL grant: (i.e. if the MCS is quite robust, a first non-colliding transmission is often decoded successfully, so a failed transmission could be assumed caused by a collision.).

For option2, SR collisions likely results in DTX detection at eNB, so no uplink grant is given. The UE behavior can be similar as for option 1. It could be further studied if the eNB can differentiate between receiving a collision or receiving high interference. If the eNB can differentiate, it could allocate UL resource for all the UEs sharing the collided resource separately, which helps to reduce the access delay caused by backoff after collision.

Based on above analysis, it seems option2 provides simpler and more resource efficient SR collision processing mechanism than option1. In addition, no new SR-RNTI is needed in option2.

Both options are inefficient in case PUCCH-SR collision occurs, but again, if SR period is short and few UEs share it, the collision probability remains low.

The sharing PUCCH-SR procedure is compared with CB-PUSCH and we conclude that CB-PUSCH provides best delay performance when the eNB has unused PUSCH resource. When the network is loaded, sharing SR is preferred.

FIG. 8 is an example for a diagram for performing contention based transmission.

There might be 3 ms difference between contention based transmission and 1 ms SR period with the assumption of contention based resource is available every TTI, because UE does not need to transmit the dedicated-SR (D-SR) and wait for eNB to response. The same performance could be achieved with dedicated pre-allocation, but it would be very costly to allocate dedicated resources for every UE in every TTI. SR associated contention based transmission offers an interesting compromise, in which the pre-allocated resource is shared and identification of the UEs making use of it is done via the D-SR. The basic procedure of SR associated contention based transmission is shown in FIG. 8.

The eNB configures the UE with D-SR and shared resource (S801). Upon UL data arrival, UE sends SR and the TB on the shared resource “simultaneously” without waiting for dedicated UL grant (S803). The eNB can identify the UEs using the contention based resource based on the received SR. When eNB receives more than one SR linked to same resource, which means collision happens, ACK the TB no matter it is correctly decoded or not and give dedicated grant to each UE sent the SR, i.e. fall back to R8/9; (The ACKed TB would rely on RLC retransmission for collision case). When the eNB only receives one SR linked to the same resource, no collision, NACK if the TB is not correctly decoded, ACK otherwise. Thus from UE perspective, normal R8/9 HARQ is still applicable (S805).

Adaptive retransmission with different resource is possible as UE is identified with the SR which will reduce the load on the contention based resource (S807).

Meanwhile, resource usage efficiency was one of the main concerns raised on contention based transmission on PUSCH because very conservative MCS needs to be used to guarantee the coverage. A contention based resource for a TB of the mostly mentioned typical TCP ACK use case, taking into account RLC header (at least 1˜2 bytes)+MAC header with one more byte to be added for UE identity and possible BSR (2˜4 bytes), would need 3˜4 PRBs (16 bits TBS for one PRB with most conservative MCS); while if dedicated grant with proper MCS (at most 712 bits TBS for one PRB), much less resource is needed to accommodate the TB. If several contention based resources are to be reserved to reduce collision probability, the capacity for dedicated grant would be significantly impacted, which makes the 3 ms latency reduction optimization a rather expensive one.

Furthermore, normal HARQ operation probably cannot work because retransmission from UE upon receiving NACK does not help or even makes the situation worse if the decoding failure is because of collision, and it is difficult (if not impossible) for the eNB to make soft combining of the TB transmitted on the contention based resource; while ACK cannot be interpreted as ACK because it could be an ACK for other UEs. No HARQ makes resource efficiency worse as even more conservative MCS would be required to ensure the TB can be decoded within only one transmission for cell edge UE.

FIG. 9 is a diagram for MAC structure overview in a UE side.

The MAC layer handles logical-channel multiplexing, hybrid-ARQ retransmissions, and uplink and downlink scheduling. It is also responsible for multiplexing/demultiplexing data across multiple component carriers when carrier aggregation is used.

The MAC provides services to the RLC in the form of logical channels. A logical channel is defined by the type of information it carries and is generally classified as a control channel, used for transmission of control and configuration information necessary for operating an LTE system, or as a traffic channel, used for the user data. The set of logical channel types specified for LTE includes Broadcast Control Channel (BCCH), Paging Control Channel (PCCH), Common Control Channel (CCCH), Dedicated Control Channel (DCCH), Multicast Control Channel (MCCH), Dedicated Traffic Channel (DTCH), Multicast Traffic Channel (MTCH).

From the physical layer, the MAC layer uses services in the form of transport channels. A transport channel is defined by how and with what characteristics the information is transmitted over the radio interface. Data on a transport channel is organized into transport blocks. In each Transmission Time Interval (TTI), at most one transport block of dynamic size is transmitted over the radio interface to/from a terminal in the absence of spatial multiplexing. In the case of spatial multiplexing (MIMO), there can be up to two transport blocks per TTI.

Associated with each transport block is a Transport Format (TF), specifying how the transport block is to be transmitted over the radio interface. The transport format includes information about the transport-block size, the modulation-and-coding scheme, and the antenna mapping. By varying the transport format, the MAC layer can thus realize different data rates. Rate control is therefore also known as transport-format selection.

To support priority handling, multiple logical channels, where each logical channel has its own RLC entity, can be multiplexed into one transport channel by the MAC layer. At the receiver, the MAC layer handles the corresponding demultiplexing and forwards the RLC PDUs to their respective RLC entity for in-sequence delivery and the other functions handled by the RLC. To support the demultiplexing at the receiver, a MAC is used. To each RLC PDU, there is an associated sub-header in the MAC header. The sub-header contains the identity of the logical channel (LCID) from which the RLC PDU originated and the length of the PDU in bytes. There is also a flag indicating whether this is the last sub-header or not. One or several RLC PDUs, together with the MAC header and, if necessary, padding to meet the scheduled transport-block size, form one transport block which is forwarded to the physical layer.

In addition to multiplexing of different logical channels, the MAC layer can also insert the so-called MAC control elements into the transport blocks to be transmitted over the transport channels. A MAC control element is used for inband control signaling—for example, timing-advance commands and random-access response. Control elements are identified with reserved values in the LCID field, where the LCID value indicates the type of control information.

Furthermore, the length field in the sub-header is removed for control elements with a fixed length.

The MAC multiplexing functionality is also responsible for handling of multiple component carriers in the case of carrier aggregation. The basic principle for carrier aggregation is independent processing of the component carriers in the physical layer, including control signaling, scheduling and hybrid-ARQ retransmissions, while carrier aggregation is invisible to RLC and PDCP. Carrier aggregation is therefore mainly seen in the MAC layer, where logical channels, including any MAC control elements, are multiplexed to form one (two in the case of spatial multiplexing) transport block(s) per component carrier with each component carrier having its own hybrid-ARQ entity.

FIG. 10 is a diagram for Scheduling Request transmission.

The scheduler needs knowledge about the amount of data awaiting transmission from the terminals to assign the proper amount of uplink resources. Obviously, there is no need to provide uplink resources to a terminal with no data to transmit as this would only result in the terminal performing padding to fill up the granted resources. Hence, as a minimum, the scheduler needs to know whether the terminal has data to transmit and should be given a grant. This is known as a scheduling request.

A scheduling request is a simple flag, raised by the terminal to request uplink resources from the uplink scheduler. Since the terminal requesting resources by definition has no PUSCH resource, the scheduling request is transmitted on the PUCCH. Each terminal can be assigned a dedicated PUCCH scheduling request resource, occurring every nth subframe. With a dedicated scheduling-request mechanism, there is no need to provide the identity of the terminal requesting to be scheduled as the identity of the terminal is implicitly known from the resources upon which the request is transmitted.

When data with higher priority than already existing in the transmit buffers arrives at the terminal and the terminal has no grant and hence cannot transmit the data, the terminal transmits a scheduling request at the next possible instant, as illustrated in FIG. 15. Upon reception of the request, the scheduler can assign a grant to the terminal. If the terminal does not receive a scheduling grant until the next possible scheduling-request instant, then the scheduling request is repeated. There is only a single scheduling-request bit, irrespective of the number of uplink component carriers the terminal is capable of. In the case of carrier aggregation, the scheduling request is transmitted on the primary component carrier, in line with the general principle of PUCCH transmission on the primary component carrier only.

The use of a single bit for the scheduling request is motivated by the desire to keep the uplink overhead small, as a multi-bit scheduling request would come at a higher cost. A consequence of the single-bit scheduling request is the limited knowledge at the eNodeB about the buffer situation at the terminal when receiving such a request. Different scheduler implementations handle this differently. One possibility is to assign a small amount of resources to ensure that the terminal can exploit them efficiently without becoming power limited. Once the terminal has started to transmit on the UL-SCH, more detailed information about the buffer status and power headroom can be provided through the inband MAC control message, as discussed below. Knowledge of the service type may also be used—for example, in the case of voice the uplink resource to grant is preferably the size of a typical voice-over-IP package. The scheduler may also exploit, for example, path-loss measurements used for mobility and handover decisions to estimate the amount of resources the terminal may efficiently utilize.

An alternative to a dedicated scheduling-request mechanism would be a contention-based design. In such a design, multiple terminals share a common resource and provide their identity as part of the request. This is similar to the design of the random access.

The number of bits transmitted from a terminal as part of a request would in this case be larger, with the correspondingly larger need for resources. In contrast, the resources are shared by multiple users. Basically, contention-based designs are suitable for a situation where there are a large number of terminals in the cell and the traffic intensity, and hence the scheduling intensity, is low. In situations with higher intensities, the collision rate between different terminals simultaneously requesting resources would be too high and lead to an inefficient design.

Although the scheduling-request design for LTE relies on dedicated resources, a terminal that has not been allocated such resources obviously cannot transmit a scheduling request. Instead, terminals without scheduling-request resources configured rely on the random-access mechanism. In principle, an LTE terminal can therefore be configured to rely on a contention-based mechanism if this is advantageous in a specific deployment.

The Scheduling Request (SR) is used for requesting UL-SCH resources for new transmission. When an SR is triggered, it shall be considered as pending until it is cancelled. All pending SR(s) shall be cancelled and sr-ProhibitTimer shall be stopped when a MAC PDU is assembled and this PDU includes a BSR which contains buffer status up to (and including) the last event that triggered a BSR, or when the UL grant(s) can accommodate all pending data available for transmission.

If an SR is triggered and there is no other SR pending, the UE may set the SR_COUNTER to 0.

As long as one SR is pending, if no UL-SCH resources are available for a transmission in this TTI, the UE may initiate a Random Access procedure on a PCell and cancel all pending SRs if the UE has no valid PUCCH resource for SR configured in any TTI.

Else if the UE has a valid PUCCH resource for SR configured for this TTI and if this TTI is not part of a measurement gap and if sr-ProhibitTimer is not running, if SR_COUNTER<dsr-TransMax, the UE may increment SR_COUNTER by 1, instruct the physical layer to signal the SR on PUCCH, and start the sr-ProhibitTimer.

if SR_COUNTER≧dsr-TransMax, the UE may notify RRC to release PUCCH/SRS for all serving cells, clear any configured downlink assignments and uplink grants, and initiate a Random Access procedure on the PCell and cancel all pending SRs.

FIG. 11 is a diagram for signaling of buffer status and power-headroom reports.

Terminals that already have a valid grant obviously do not need to request uplink resources. However, to allow the scheduler to determine the amount of resources to grant to each terminal in future subframes, information about the buffer situation and the power availability is useful, as discussed above. This information is provided to the scheduler as part of the uplink transmission through MAC control element. The LCID field in one of the MAC subheaders is set to a reserved value indicating the presence of a buffer status report, as illustrated in FIG. 11.

From a scheduling perspective, buffer information for each logical channel is beneficial, although this could result in a significant overhead. Logical channels are therefore grouped into logical-channel groups and the reporting is done per group. The buffer-size field in a buffer-status report indicates the amount of data awaiting transmission across all logical channels in a logical-channel group.

The Buffer Status Reporting (BSR) procedure is used to provide a serving eNB with information about the amount of data available for transmission in the UL buffers of the UE. RRC may control BSR reporting by configuring the two timers periodicBSR-Timer and retxBSR-Timer and by, for each logical channel, optionally signalling Logical Channel Group which allocates the logical channel to an LCG (Logical Channel Group).

For the Buffer Status reporting procedure, the UE may consider all radio bearers which are not suspended and may consider radio bearers which are suspended. A Buffer Status Report (BSR) may be triggered if any of the following events occur: i) arrival of data with higher priority than currently in the transmission buffer-that is, data in a logical-channel group with higher priority than the one currently being transmitted—as this may impact the scheduling decision, (i,e., UL data, for a logical channel which belongs to a LCG, becomes available for transmission in the RLC entity or in the PDCP entity and either the data belongs to a logical channel with higher priority than the priorities of the logical channels which belong to any LCG and for which data is already available for transmission, or there is no data available for transmission for any of the logical channels which belong to a LCG, in which case the BSR is referred below to as “Regular BSR”; retxBSR-Timer expires and the UE has data available for transmission for any of the logical channels which belong to a LCG, in which case the BSR is referred below to as “Regular BSR”) ii) change of serving cell, in which case a buffer-status report is useful to provide the new serving cell with information about the situation in the terminal, iii) Periodically as controlled by a timer (i.e.,periodicBSR-Timer expires, in which case the BSR is referred below to as “Periodic BSR”), iv) instead of padding. If the amount of padding required to match the scheduled transport block size is larger than a buffer-status report, a buffer-status report is inserted. Clearly it is better to exploit the available payload for useful scheduling information instead of padding if possible (i.e., UL resources are allocated and number of padding bits is equal to or larger than the size of the Buffer Status Report MAC control element plus its subheader, in which case the BSR is referred below to as “Padding BSR”).

For Regular BSR, if the BSR is triggered due to data becoming available for transmission for a logical channel for which logicalChannelSR-ProhibitTimer is configured by upper layers, if not running, the MAC entity starts the logicalChannelSR-ProhibitTimer. Else, if running, the MAC entity stops the logicalChannelSR-ProhibitTimer.

For Regular and Periodic BSR, if more than one LCG has data available for transmission in the TTI where the BSR is transmitted, the UE may report Long BSR. If else, the UE may report Short BSR.

If the Buffer Status reporting procedure determines that at least one BSR has been triggered and not cancelled, if the UE has UL resources allocated for new transmission for this TTI, the UE may instruct the Multiplexing and Assembly procedure to generate the BSR MAC control element(s), start or restart periodicBSR-Timer except when all the generated BSRs are Truncated BSRs, and start or restart retxBSR-Timer.

Else if a Regular BSR has been triggered and logicalChannelSR-ProhibitTimer is not running, if an uplink grant is not configured or the Regular BSR was not triggered due to data becoming available for transmission for a logical channel for which logical channel SR masking (logicalChannelSR-Mask) is setup by upper layers, a Scheduling Request shall be triggered.

A MAC PDU may contain at most one MAC BSR control element, even when multiple events trigger a BSR by the time a BSR can be transmitted in which case the Regular BSR and the Periodic BSR shall have precedence over the padding BSR.

The MAC entity may restart retxBSR-Timer upon indication of a grant for transmission of new data on any UL-SCH.

All triggered BSRs may be cancelled in case UL grants in this subframe can accommodate all pending data available for transmission but is not sufficient to additionally accommodate the BSR MAC control element plus its subheader. All triggered BSRs shall be cancelled when a BSR is included in a MAC PDU for transmission.

The MAC entity shall transmit at most one Regular/Periodic BSR in a TTI. If the UE is requested to transmit multiple MAC PDUs in a TTI, it may include a padding BSR in any of the MAC PDUs which do not contain a Regular/Periodic BSR.

All BSRs transmitted in a TTI always reflect the buffer status after all MAC PDUs have been built for this TTI. Each LCG shall report at the most one buffer status value per TTI and this value shall be reported in all BSRs reporting buffer status for this LCG.

As mention above, when uplink data becomes available for transmission in the UE side but has no uplink grant, the UE will trigger SR due to triggered BSR and transmit BSR/uplink data when the UE gets uplink grant from the eNB. This ensures that only one UE uses the provided uplink grant at the cost of latency which is brought up by the sequential procedure of getting uplink grant from the eNB.

In order to reduce latency in uplink data transmission, two approaches have been considered in 3GPP: i) contention based SR transmission, and ii) contention based PUSCH transmission.

The contention based SR transmission enables more frequent transmission of SR by configuring SR with shorter SR period. Hence, the UE can inform the eNB of need for uplink grant as soon as possible if SR is successfully transmitted. However, this inevitably comes with possible contention in SR transmission, which may in return delays getting uplink grant.

The contention based PUSCH transmission allows UE to transmit uplink data by using the pre-configured uplink grants which can be allocated to multiple UEs. This of course enables the UE to transmit uplink data as soon as the uplink data becomes available for transmission. However, it requires the contention resolution methods in case contention occurs, e.g., transmitting SR along with PUSCH, in order for the eNB to provide another uplink grant to the UE.

For both approaches, once the UE gets uplink grant from the eNB the UE will report the buffer status to the eNB so that sufficient uplink grant can be received by the UE as in the legacy, meaning that the latency still exists.

FIG. 12 is a conceptual diagram for receiving a MAC CE for contention-based PUSCH in a wireless communication system according to embodiments of the present invention.

In this invention, the UE transmits buffer status information and a UE identifier by using a contention based time/frequency resource, i.e., CB grant, to the eNB. When the eNB receives the buffer status information and the UE identifier on the CB grant, the eNB identifies the UE which transmitted the buffer status information and the UE identifier on CB grant. The eNB provides an uplink grant to the UE of which UE identifier is identified by the eNB. In addition, the eNB sends a Contention-based MAC CE by including the UE identifier of the successfully identified UE in order to inform the UEs using the same CB grant of the successfully identified UE on the CB grant.

The eNB configures a UE with i) at least one Contention Based grant (CB grant), ii) Contention based RNTI, i.e., CB-RNTI, and iii) an UE identifier (S1201).

The CB grant means a grant configured to the UE with an opportunity to send UL packets without requiring the UE to send a scheduling request. The CB grant is a time-frequency resource on which at least two UEs are allowed to transmit a MAC PDU to the eNB. That is, Multiple UEs can be configured with the same CB grant, i.e., the eNB can configure multiple UEs with the same CB grant.

Preferably, the CB grant is used only for synchronized UEs in order to transmit uplink data without sending Scheduling Request in advance.

Preferably, the CB grant correspond to uplink resources only that have not been reserved for dedicated uplink transmission (i.e., the dedicated uplink transmission is performed using conventional uplink grant) so that transmissions based on the CB grant do not interfere with dedicated uplink transmission.

Preferably, the size of CB grant is such that buffer status information and a UE identifier are included in the CB grant.

The CB-RNTI is used for monitoring PDCCH by the UE for receiving Contention-based MAC control element.

Preferably, there is a 1 to 1 mapping between the CB grant and the CB-RNTI. That is, for one CB grant, there is one CB-RNTI associated with that CB grant. For one CB-RNTI, there is one CB grant associated with that CB-RNTI.

Preferably, the CB-RNTI is either configured by the eNB or obtained/calculated by the UE based on the CB grant.

The UE identifier identifies the UE among the UEs which are configured with the same CB grant.

If the UE has data to transmit to the eNB and if the UE has at least one CB grant configured for the UE, the UE generates a MAC PDU including an identifier of the UE on the CB grant and transmits the generated MAC PDU to the eNB by using one of the CB grant(s) configured for the UE (S1203).

Preferably, the MAC PDU further includes buffer status information if there is no UL grant sufficiently.

If the eNB receives the MAC PDU on the CB grant (S1203), the eNB identifies which UE transmitted the MAC PDU on the CB grant by checking the UE identifier included in the received MAC PDU (S1205). To the identified UE, the eNB provides an uplink grant which is used only by the identified UE for transmitting data to the eNB (S1207). The PDCCH indicating the uplink grant is addressed by the C-RNTI of the identified UE. The eNB generates a Contention-based MAC CE (CB MAC CE) by including the UE identifier of the identified UE for the MAC PDU received on the CB grant, and transmits the generated CB MAC CE by using the PDCCH addressed by the CB-RNTI associated with the CB grant (S1209).

By the way, the UE starts monitoring PDCCH addressed by CB-RNTI associated with the CB grant after transmitting the generated MAC PDU by using the CB grant (S1211).

At that time, when the UE receives the uplink grant in response to the MAC PDU including buffer status information transmission, the UE performs uplink transmission as in the legacy according to spec. TS36.321 (S1213).

When the UE receives the CB MAC CE indicated by the PDCCH addressed to the CB-RNTI, the UE stops monitoring PDCCH addressed by CB-RNTI, and checks whether the UE identifier of the UE is included in the CB MAC CE or not (S1215).

If the UE identifier of the UE is not included in the CB MAC CE, the UE considers that the data transmission on the CB grant associated with the CB-RNTI as not successful (S1217). In the step of S1217, the UE generates a MAC PDU by including buffer status information and the UE identifier in the MAC PDU, and transmits the generated MAC PDU to the eNB by using one of the CB grant(s) configured for the UE. And the UE starts monitoring PDCCH addressed by CB-RNTI associated with the CB grant after transmitting the generated MAC PDU by using the CB grant.

If the UE identifier of the UE is included in the CB MAC CE, the UE does nothing, i.e., the UE starts monitoring a PDCCH addressed to a C-RNTI for uplink grant and performs uplink transmission by using the received uplink grant as in the legacy according to spec. TS36.321 (S1219).

The embodiments of the present invention described herein below are combinations of elements and features of the present invention. The elements or features may be considered selective unless otherwise mentioned. Each element or feature may be practiced without being combined with other elements or features. Further, an embodiment of the present invention may be constructed by combining parts of the elements and/or features. Operation orders described in embodiments of the present invention may be rearranged. Some constructions of any one embodiment may be included in another embodiment and may be replaced with corresponding constructions of another embodiment. It is obvious to those skilled in the art that claims that are not explicitly cited in each other in the appended claims may be presented in combination as an embodiment of the present invention or included as a new claim by subsequent amendment after the application is filed.

In the embodiments of the present invention, a specific operation described as performed by the BS may be performed by an upper node of the BS. Namely, it is apparent that, in a network comprised of a plurality of network nodes including a BS, various operations performed for communication with an MS may be performed by the BS, or network nodes other than the BS. The term ‘eNB’ may be replaced with the term ‘fixed station’, ‘Node B’, ‘Base Station (BS)’, ‘access point’, etc.

The above-described embodiments may be implemented by various means, for example, by hardware, firmware, software, or a combination thereof.

In a hardware configuration, the method according to the embodiments of the present invention may be implemented by one or more Application Specific Integrated Circuits (ASICs), Digital Signal Processors (DSPs), Digital Signal Processing Devices (DSPDs), Programmable Logic Devices (PLDs), Field Programmable Gate Arrays (FPGAs), processors, controllers, microcontrollers, or microprocessors.

In a firmware or software configuration, the method according to the embodiments of the present invention may be implemented in the form of modules, procedures, functions, etc. performing the above-described functions or operations. Software code may be stored in a memory unit and executed by a processor. The memory unit may be located at the interior or exterior of the processor and may transmit and receive data to and from the processor via various known means.

Those skilled in the art will appreciate that the present invention may be carried out in other specific ways than those set forth herein without departing from essential characteristics of the present invention. The above embodiments are therefore to be construed in all aspects as illustrative and not restrictive. The scope of the invention should be determined by the appended claims, not by the above description, and all changes coming within the meaning of the appended claims are intended to be embraced therein.

INDUSTRIAL APPLICABILITY

While the above-described method has been described centering on an example applied to the 3GPP LTE system, the present invention is applicable to a variety of wireless communication systems in addition to the 3GPP LTE system. 

1. A method for a User Equipment (UE) operating in a wireless communication system, the method comprising: configuring a Contention Based (CB) grant and a CB-Radio Network Temporary Identifier (RNTI) associated with the CB grant; transmitting a first Medium Access Control (MAC) Protocol Data Unit (PDU) including an identifier of the UE using the CB grant; receiving a CB MAC Control Element (CE) indicated by a Physical Downlink Control Channel (PDCCH) addressed to the CB-RNTI in response to the MAC PDU transmission; determining that the first MAC PDU transmission is failed if the identifier of the UE isn't included in the MAC CE; and transmitting a second MAC PDU including the identifier of the UE using the CB grant.
 2. The method according to claim 1, wherein if the identifier of the UE is included in the CB MAC CE, the UE determines that the transmission of the first MAC PDU is successful.
 3. The method according to claim 2, further comprising: starting monitoring a PDCCH addressed to a C-RNTI for uplink grant.
 4. The method according to claim 1, wherein the CB grant is a time/frequency resource on which at least two UEs are allowed to transmit a MAC PDU to the eNB.
 5. The method according to claim 1, wherein the CB-RNTI is either configured by the eNB or calculated by the UE based on the associated with CB grant.
 6. The method according to claim 1, wherein MAC PDU includes buffer status information.
 7. The method according to claim 3, wherein when the CB MAC CE is received, the UE stops monitoring the PDCCH addressed to the CB-RNTI.
 8. A User Equipment (UE) operating in a wireless communication system, the UE comprising: a Radio Frequency (RF) module; and a processor configured to: control the RF module, configure a Contention Based (CB) grant and a CB-Radio Network Temporary Identifier (RNTI) associated with the CB grant; transmit a first Medium Access Control (MAC) Protocol Data Unit (PDU) including an identifier of the UE using the CB grant; receive a CB MAC Control Element (CE) indicated by a Physical Downlink Control Channel (PDCCH) addressed to the CB-RNTI in response to the MAC PDU transmission; determine that the first MAC PDU transmission is failed if the identifier of the UE isn't included in the MAC CE; and transmit a second MAC PDU including the identifier of the UE using the CB grant.
 9. The UE according to claim 8, wherein if the identifier of the UE is included in the CB MAC CE, the UE determines that the transmission of the first MAC PDU is successful.
 10. The UE according to claim 9, wherein the UE starts monitoring a PDCCH addressed to a C-RNTI for uplink grant.
 11. The UE according to claim 9, wherein the CB grant is a time/frequency resource on which at least two UEs are allowed to transmit a MAC PDU to the eNB.
 12. The UE according to claim 9, wherein the CB-RNTI is either configured by the eNB or calculated by the UE based on the associated with CB grant.
 13. The UE according to claim 9, wherein MAC PDU includes buffer status information.
 14. The UE according to claim 10, wherein when the CB MAC CE is received, the UE stops monitoring the PDCCH addressed to the CB-RNTI.
 15. A method for a User Equipment (UE) operating in a wireless communication system, the method comprising: receiving a Contention Based (CB) grant from an eNB; if the UE has data to transmit to the eNB, generating a MAC PDU including an identifier of the UE and buffer status information without the data; transmitting the generated MAC PDU to the eNB using the CB grant; and monitoring a PDCCH addressed by C-RNTI for receiving UL grant to be used for transmitting the data.
 16. The method according to claim 15, wherein the buffer status information includes an amount of the data.
 17. The method according to claim 15, further comprising: receiving a UL grant; and transmitting the data using the received UL grant to the eNB.
 18. The method according to claim 15, wherein the CB grant is a time/frequency resource on which at least two UEs are allowed to transmit a MAC PDU to the eNB.
 19. The method according to claim 15, further comprising: monitoring a PDCCH addressed by CB-RNTI for receiving a CB MAC CE including the identifier of the UE. 